1. Field of the Invention
This invention pertains to surface acoustic wave devices and more particularly relates to an improved surface acoustic wave device wherein the improvement comprises a means for mounting the acoustic substrate to provide vibration and shock isolation and to permit thermal expansions and contractions of the acoustic substrate without creating stresses or strains therein.
2. Description of the Prior Art
Surface acoustic wave (SAW) devices must meet full operational performance requirements during and after the acceleration, impact, shock and vibration conditions that occur in spacecraft, projectile, and missile environments. Furthermore, the differing thermal expansion characteristics of the acoustic substrate and the base on which it is mounted must be allowed for; otherwise stresses may develop in the acoustic substrate that may lead to a deterioration of performance and device failure. Mounting structures well known in the art have been used with varying degrees of success as the most cost effective method of minimizing both thermally and mechanically induced stresses. The reliability of surface acoustic wave devices, particularly in hostile environments is therefore directly dependent on the mounting structure utilized.
It is also important to use mounting techniques which do not contaminate the surface of the SAW device. Contaminants on the surface of the acoustic substrate may result in a mass loading effect which alters the frequency of the device, causing device aging.
One mounting technique known in the art comprises gluing the acoustic substrate directly to a base or header. A variation of this method employs flexible epoxy or flexible epoxy impregnated cloth tape to mount the acoustic substrate to the base. Unlike the first method, this latter method allows for thermal expansion. The high outgassing of flexible expoxies, however, may contaminate the acoustic substrate resulting in increased aging effects. Another mounting method known in the art is a cantilevered scheme comprising bonding the acoustic substrate on one end only to, for example, a TO-8 type header base, with a hold down clearance clamp over the opposite end to dampen vibrations. This method also allows for temperature changes. These methods, however, do not provide sufficient shock and vibration isolation for reliable operation under exposure to severe mechanical transients.
Two mounting techniques known by workers in the bulk mode crystal resonator field, and adopted for use with SAW devices are shown in FIGS. 1 and 1A. FIG. 1 shows a C-clip 10 mounted to a SAW acoustic substrate 11 and base 30 using epoxy resin 12 as an adhesive. These C-clips are typically manufactured from gold ribbon and electroplated with nickel. In FIG. 1A, a mounting clip known in the art as an L-clip 13 is shown attached to SAW substrate 11 using epoxy resin 12.
C-clips and L-clips are however, primarily flexible in only one direction, as shown by arrow 14 in FIGS. 1 and 1A. Shocks in other directions will be transmitted to the acoustic substrate causing stresses therein, and may also deform the clips. As the deformation in the clips is relaxed, additional stresses are created in the acoustic substrate.
Important requirements for a high shock mounting structure are that it not apply undue stress to the acoustic substrate and that it be strong enough not to permanently deform under the loading experienced during shock. The C and L-clip mounted SAW devices are known to have mounting stresses which increase device aging. A stress free mount will serve to reduce these effects.
The present invention provides a simple method for mounting the acoustic substrate of surface acoustic wave devices to minimize the effects of shock and vibration, and permit thermal expansions and contractions without causing undue stress which leads to unreliable operation and increased aging effects.